1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for ringing a telephonic device.
2. Description of the Related Art
In communications systems, particularly telephony, it is a common practice to trans-mit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office through a subscriber line. At the subscriber end, a telephonic device may be employed to establish communication with a remote user using the subscriber line. The combination of the telephonic device and the subscriber line is commonly referred to as a subscriber loop.
A line card generally includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The SLIC interfaces with the subscriber loop, and the SLAC interfaces with the SLIC. The SLIC and the SLAC carry out the well-known BORSCHT (Battery feed, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, and Test) functions.
Typically, when an end user initiates a call, the line card provides a ringing AC ringing signal and, often, a DC bias signal, to the subscriber loop to ring the telephonic device. In the United States, the AC ringing signal generally varies from a 16 Hz to 66⅔ Hz, although a 20 Hz signal is commonly used. Other countries may employ a ringing signal of a different frequency than that of the ringing signal employed in the United States. For example, in European countries, the ringing signal is 25 Hz. The ringing signal can either be internally or externally generated.
The ringing signals generally tend to be larger signals than the signals utilized for normal voice operations (i.e., during non-ringing mode). For example, during non-ringing mode, the voltage of the signals is generally no more than 50 volts DC, when no current is flowing, In contrast, the ringing signal may be a 80 volt-rms signal that is capable of saturating the voice components of the line card.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.